A320 Fuel Leak scenario

[Boyington]

Can someone please explain how to determine Fuel Leak in the scenario given below:
The flight originated with 18,400 kgs of fuel. After 2 hrs of flying there was a shortage of 200 kgs i.e when we added the fuel consumed and the FOB(say X) and subtracted the value (X) from the fuel at departure it was short by 200kgs. The difference progressively increased to 400kgs. And an imbalance developed in the sense that the left tanks were 300 kgs less than the right tanks. When we tried to calculate the leak rate we couldn't, as the fuel consumed by the engines over a fixed period of time were the same as that was being reduced from the tanks.
In this case how do I determine where the leak is from?

[safelife]

From the engine, behind the fuel metering unit, because that's where fuel flow is measured.

[vilas]

The fuel leak procedure does not tell you to calculate leak rate. There is a difference between leak rate and depletion rate. In your case fuel leak is confirmed from left side. So first thing as per procedure is LAND ASAP. To do that consider left side fuel as lost and select suitable diversion and head towards it. Only thing left to find out is whether it is from left tank or left engine. For that apply the procedure from ONE INNER TANK DEPLETES FASTER THAN THE BY AT LEAST 300KGS IN LESS THAN 30 MINUTES. It asks you to shut down the engine and if the left tank quantity remains static then it is from engine and has stopped. So the fuel can be balanced and used. If quantity keeps decreasing then it is from the tank so restart the engine but do not balance. Let the engine run till it flames out.

[Boyington]

Thanks Vilas. I did not divert as the leak was very small and there were three airports in the vicinity and we were approaching our destination too. Is there any other way to deter if fuel leak is from the tank or from the engine, without shutting down the engine.

[SeenItAll]

Look outside and see if you see fuel streaming from the wing or from a nacelle?

[FlightDetent]

FQI and their accuracy is some serious black magic. To confirm you actually had a leak and not just misbehaving readings a couple of additional steps may be necessary. Did you take any?

To illustrate I used to fly particular MSN which seem to show a constant leak from whatever had been the FOB down to about 4000 kgs, at which point the fuel magically re-appeared. After one cautious en-route diversion as a result of this, and .... plus .... with ...., it turned out that all was within the FQI specification. Nicely explained in the old (discontinued) blue FCOM bulletin: FQI Accuracy.

[safelife]

You can't really tell if it's from the engine or not, until you shut down the engine. As Vilas illustrated, this isn't called for in your case, as the leak rate isn't high enough.
I would hazard a guess that it was a FQI problem. In A330/340 it's quite normal to have up to 700 kg less during some stage of the flight comparing fuel on board + fuel used with total fuel on departure.
Water in the tank can be one reason. Remaining fuel and fuelled fuel with different densities mixing slowly another.

[vilas]

Other than visible leak from the engine, fuel leak that is down stream of the fuel flow meter will be accounted for but that fuel flow will be much higher than the other one.

[Uplinker]

Our 321s 'hide' about 200kg of fuel during the cruise, but our 320s don't. The 321s have jet centre tank pumps, the 320s have electric centre tank pumps - I don't know if this is the reason for the hiding behaviour?

I have also noticed that the FQI system can give an incorrect total value when tank contents are transferring. For example when the outer wing tank valves open and transfer their contents to the inner tanks.

I royally cocked up a fuel leak in the SIM by asking CC to go and view the source of the leak. Wasted a lot of time and fuel - and I did not get a clear answer. Best course of action seems to be to ignore that first part of the QRH drill and jump to the part of the drill that assumes the source of the leak is not determined?

[Denti]

I have seen both A320/A321 "hide" up to the 400kgs of fuel that during later stages of the flight reappeared. Especially on longer flights. To be honest, didn't check the pump configuration as we do have a few A320s with the jet pump to main tank configuration, same as on A321s.

[FlightDetent]

Same here, it was an A321, MSN 990ish, now scrapped. And I actually found a file with values I marked down then

1674

The amount "missing" would increase until about 8t FOB, then come slowly reappear and by 4000 kg all was back. The more you had at the beginning, the more was missing by the time you were down to 8000. Ad extremis 900 kgs!

[FullWings]

I think quite a few aircraft types do similar things to this. When you think of all the sloshing around, density gradients, sensor tolerances, etc. it’s a wonder they’re as accurate as they are.

If I was designing one I think I’d err on the side of caution and display the lowest sensible reading that you could get from the raw data. The 777 seems to work that way as I’ve often seen a “dip” mid-flight but never excess. It does seem to get much more accurate as the fuel amount reduces, which on reflection is probably the best way round...

[Superpilot]

Same here!

A321 MSN 10xx. For us it happened more during latter stages of flight. Though occasionally I saw it during mid flight on a 5hr sector. Example: TOD we can be showing EFOB 2.5t but starts going down about 0.1t every 5 minutes. When we land we see a figure closer to 2.0t being actual FOB. Refueling figures verify there was no "leak". The system was simply getting it wrong. Reported to engineering who thought I was smoking something (JET A1 probably)

[Goldenrivett]

Airbus A-319 had fuel contents in error of over 1Ton and flamed out one engine. Not noticed during refuelling operations because all the previous uplifts were within tolerance.
Incident: Air France A319 at Paris on Mar 12th 2014, engine starves due to lack of fuel (http://avherald.com/h?article=4744fa10)

[Lantirn]

Checking the FOB and FU if possible when lined up in the Rwy is the best approach to check for FQI errors later on!

[Chrome]

I had a similar case. FOB after refueling was what I ordered for a 3 hour flight.

During the climb phase, ECAM Caution came on for a F.Used/FOB Disagree - Fuel Leak Proc Apply. We lost about 1000kg. We went through the QRH and fuel leak source was inconclusive. We stopped at the part where we were asked to shut down engines as we were still 15 minutes away from the point of departure and fuel missing was down to 600kg. After 30 minutes, it became 400kg. So we decided to get to cruise level and monitor further. Short story we got back to FOB at departure after 10 minutes in cruise.

Not sure what happened there but it was my first time. Could it be water in the fuel?

[FlightDetent]

Chrome, that does not sound right.

There was a lovely FCOM blue FQI Accuracy bulletin attached to the old FCOM3, explaining in some detail all the black magic that FOB and FU measurements are. However, nothig in there to justify your experience.

[Uplinker]

@Chrome, With my electronics hat on, my guess would be a bad connection somewhere in the FQI system. Bad electrical connections can be very temperature sensitive* and this could easily have been a duff joint that went bad as it went through a particular temperature but recovered as the temperature changed.

Or it could have been that fuel contents were transferring. On Airbus I sometimes notice FQI errors while this happens but when the transfer stops the FQI reads correctly again. Having said that, the most I have seen on the A320 family is 400kg temporarily missing.

*Indeed, temperature changes are used to find duff joints when trouble shooting. We use freezer spray and heat guns to target suspect joints.

[gusting_45]

I havd had two similar experiences to Chrome's.

First one was in the cruise to Tallinn. It appeared that all of the fuel had disappeared from the LH Outer tank, and no it had not transferred to the Inner. No leak from the LH Inner so flight continued to destination as the remaining was sufficient if a little skinny. During the descent the fuel magically re-appeared. Fuel sensor problem in the tank.

Second was very similar, bizarrely also during the cruise on a flight to Tallinn. It appeared that we had a significant fuel loss from the LH Inner this time. Applied the procedure to check Fuel uplift vs burn and fuel remaining etc. Fuel could not be accounted for, subsequent diversion ensued followed by engineering inspection. Ground engineers pumped all of the fuel on board to the RH wing tanks. Magically, all the fuel re-appeared. There was no fuel leak. Once again, fuel quantity (more correctly I believe, Mass sensors) in the LH Inner wing tank were faulty.

[Chrome]

Chrome, that does not sound right.
After the incident I did have a re-read of the bulletin to get answers. It really doesn't explain my experience does it. 10 years on the bus, I have never seen it before. I even flew the same aircraft the very next day, it didn't reoccur.

Chrome, With my electronics hat on, my guess would be a bad connection somewhere in the FQI system.
What was also different was that the fuel temperature in all tanks were above 20°C at FL360. That could be the explanation.